A survey of locations at a survey site may involve use of location coordinates from previously-surveyed locations ("control survey points"), such as recognizable landmarks and other survey monuments, together with generation of location coordinates for newly-surveyed locations. Even where location coordinates for only newly-surveyed locations are used, these coordinates may not be fully consistent with each other when corrections are made for any errors introduced by use of a local coordinate system. The location coordinates of these locations, representing newly-surveyed locations and/or previously-surveyed locations, must be made consistent with each other in some "best fit" sense.
Survey of a chosen region often requires use of a globally defined survey ellipsoid, such as the NAD27, NAD83 or WGS84 ellipsoid, and of a locally defined ellipsoid that takes account of the local terrain. A globally defined survey ellipsoid is intended to provide a best fit, in some quantitative sense, to the terrain for the entire Earth. A locally defined ellipsoid, by contrast, is only intended to provide a best fit for a locally defined region and thus may provide a better fit over this limited region.
In a survey of a local region, location coordinates may first be obtained using a first, globally defined datum C1, after which these coordinates are re-expressed in terms of a second datum C2, which may be globally defined or locally defined, using a coordinate transformation T. Accurate coordinates for a plurality of survey control points are often available, and these survey control point coordinates are often used to "anchor" the transformation of location coordinates for other survey points. Coordinate differences between a point p(x,y,z) in the first, globally defined datum C1 and the corresponding point p'(x',y',z') in the second datum C2 are presently determined, using a point-by-point approach in which the group of survey control points chosen for "anchoring" can vary with the local terrain. FIG. 1 illustrates a vector difference .DELTA.r=(x'--x, y'--y, z'--z) between point coordinates (x,y,z) on a surface S1, defined using the datum C1, and the corresponding point coordinates (x',y',z') on a corresponding surface S2, defined using the datum C2. In the past, these coordinate differences were determined point-by-point on each of a plurality of sub-regions or patches that together make up a survey region SR, and these differences were often not continuous or consistent in moving from one sub-region to a contiguous sub-region. Thus, the coordinate differences had to be recomputed by applying the transformation T to each new point in the first datum; and parameters defining the transformation T might change in moving from one sub-region to a contiguous sub-region.
Several workers have considered the problems of representation of locations on a map or chart display and/or adjustment of location coordinates. Previous workers in this field often assume that the coordinates for locations of interest in a coordinate system, or in two or more associated coordinate systems, are consistent with each other. Further, the computations and coordinate manipulations are usually performed in a post-processing environment, rather than in a real time environment in the field at the time the survey measurements are made.
What is needed is an approach that allows real time processing, or post processing, to determine and apply a survey coordinate transformation between the first datum and the second datum in a chosen region that varies continuously and consistently with the point chosen in the region. Preferably, this transformation should be optimal in some sense over a grid of survey control points, and the transformation should depend only minimally or not at all on the particular grid chosen for such optimization. Preferably, this approach should allow use of any of a variety of generally defined coordinate transformations, each with its own set of transformation parameters that are to be optimized based on the survey control constraints imposed.